The present invention relates to a method for etching a gallium nitride compound-based semiconductor. More specifically, the invention relates to a photoenhanced wet etching method for the gallium nitride compound-based semiconductor.
Gallium nitride compound-based semiconductors (for example, but not exclusively, AlxGa1xe2x88x92xN, In1xe2x88x92yGayN and GaN or a mixture thereof, where x and y are numbers between zero and one, indicating the mole fraction of Al or In) have been receiving attention as the material for blue light emitting devices, power semiconductor devices and ultraviolet photodetectors. In order to fabricate these devices using the gallium nitride compound-based semiconductors, etching technology must be developed for forming mesa structures and the like. However, gallium nitride compound-based semiconductors are highly stable materials. Therefore, the semiconductors cannot be etched near the room temperature with etchants commonly used to etch other compounds of III-V group, such as hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3), hydrofluoric acid (HF), and mixed solutions thereof.
Various methods of wet etching gallium nitride compound-based semiconductors (hereinafter called GaN type semiconductors for brevity) have been reported by other laboratories. These include: a method using a solution obtained by heating sodium hydroxide or potassium hydroxide and the like to a temperature of 800 degrees C. or more; a method using an etchant obtained by heating a mixture of phosphoric and sulfuric acid to a temperature of approximately 200 degrees C.; and the like. However, these methods employ corrosive substances at high temperatures and are not suitable for industrial use. Recently, a photoelectrochemical (PEC) etching technique has been developed, which can produce anisotropic, dopant selective or smooth etching. Moreover, mesas and pillars can be produced.
Unfortunately, PEC etching can, in general, only be used for GaN deposited on a conducting substrate (for example SiC), as the technique requires an electrical contact to be made to the sample. The contact is attached to a Pt cathode which is located remotely from the sample, but also immersed in the water-based solution. Peng et al., Appl. Phys. Lett., 72, 939 (1998), etched GaN on a sapphire (insulating) substrate, but using a specific mask, consisting of stripes of a Ti/Pt bilayer. This metal mask was again connected to a Pt counter electrode immersed in the aqueous solution. Such an arrangement can etch GaN on an insulating substrate, but cannot produce pillars or mesas since the metal mask must be completely interconnected.
Since GaN type semiconductors are often grown on insulating substrates, and since mesas and pillars are required for fabrication of various devices, these methods are limited in practical importance. Moreover, the necessity to make electrical contact to the semiconductor makes the process impractical for industrial use. The mechanism of PEC etching of GaN is thought to consist of the following steps:
1. Absorption of an ultraviolet photon, resulting in the generation of an electron-hole pair in the semiconductor. Alternatively, the electron-hole pair can be formed through thermal activation.
2. The electron (exe2x88x92) is transported through the sample""s electrical contact to the Pt counter electrode located remotely in the solution where the Pt surface is catalytic for the reaction: H++exe2x88x92xe2x86x92xc2xdH2 
3. The hole (p+) is used to form surface oxide on the GaN in the following reaction: 2GaN+3OHxe2x88x92+6p+xe2x86x92Ga2O3+N2+3H+
4. The surface oxide Ga2O3 is dissolved into the aqueous solution in the following reaction: Ga2O3+3H2Oxe2x86x922Ga3++6OHxe2x88x92, where Ga2O3 is known to be soluble in either acid or alkaline solutions.
Thus, the overall reaction associated with GaN etching is: GaNxe2x86x92Ga3++xc2xdN2.
Electrons and holes are both produced and consumed during the reaction. Water, and/or its ions, H"" and OHxe2x88x92 are also involved.
It has been found that gallium nitride compound-based semiconductors can be etched effectively by a method comprising at least the step of applying to the semiconductor an aqueous solution containing an oxidizing agent, the solution and said agent in combination, i.e. jointly, capable of dissolving gallium oxide and consuming electrons generated during etching.
The solution and the semiconductor may be irradiated with light of a wavelength selected from the visible and ultraviolet.
Preferably, the semiconductor is irradiated with light of a frequency above the band gap of the semiconductor.
The oxidizing agent may be peroxydisulfate ions. The solution may be acidic or alkaline, i.e. comprise hydroxide or hydrogen ions.
If the sample is masked by a non-catalytic metal, both the sample and solution may be irradiated with light of wavelength shorter than 260 nm such that free radicals are produced by the photochemical reaction of the peroxydisulfate ions.
The aqueous solution may be purged with an inert gas, such as N2 or Ar, to remove dissolved oxygen before any illumination is provided, and throughout the etching process.
The semiconductor is partially masked during etching with a metal, for example platinum, titanium, gold, or any other metal that is essentially resistant to etching under the gallium nitride compound-based semiconductors etching conditions.
Alternatively, the semiconductor may be partially masked during the etching with a non-metallic material, for example silicon dioxide or silicon nitride, that is essentially chemically inert under the GaN-type semiconductor etching conditions.
The gallium nitride compound-based semiconductor may be one or more of a large array of compounds, for example, but without limiting to, AlxGa1xe2x88x92xN, In1xe2x88x92yGayN and GaN.